Dual effects of the non-esterified fatty acid receptor 'GPR40' for human health

G protein-coupled receptor 40 (GPR40), a receptor for diverse non-esterified fatty acids, is expressed predominantly in the wide variety of neurons of the central nervous system and β-cells in the pancreatic islets. Since deorphanization of GPR40 in 2003, the past decade has seen major advances in our understanding of its role in the insulin secretion. However, there is still a great deal to be elucidated about the role of GPR40 in the brain, because the latter shows the most abundant GPR40 mRNA expression among the human tissues. Since a substantial expression of GPR40 is also seen in the hypothalamus, 'brain-lipid sensing' might be involved in the control of insulin secretion and energy balance. The preceding experiments using monkeys after transient global brain ischemia, have highlighted implication of GPR40 for amplifying adult hippocampal neurogenesis. Although GPR40-mediated intracellular signaling was recently found to result in phosphorylation of cAMP response element-binding protein (CREB) necessary for the neuronal differentiation and synaptic plasticity, the signaling cascade is still incompletely understood. Furthermore, in response to conjugated linoleic acids or trans isomers of arachidonic acid, GPR40 was recently demonstrated in rodents to mediate lipotoxicity to β-cells, neurons, or microvessels, which result in diabetes, retinopathy, stroke, etc. However, it still remains undetermined in humans whether and how oxidized, conjugated, or excessive fatty acids evoke lipotoxicity. Although literature about GPR40 is limited especially about the brain or the brain-pancreas interaction, this review aims at summarizing beneficial as well as detrimental effects of this receptor in the brain and pancreas in response to diverse fatty acids.